crypt

CRYPT(5)                      File Formats Manual                     CRYPT(5)

NAME
     crypt — storage format for hashed passphrases and available hashing
     methods

DESCRIPTION
     The hashing methods implemented by crypt(3) are designed only to process
     user passphrases for storage and authentication; they are not suitable
     for use as general-purpose cryptographic hashes.

     Passphrase hashing is not a replacement for strong passphrases.  It is
     always possible for an attacker with access to the hashed passphrases to
     guess and check possible cleartext passphrases.  However, with a strong
     hashing method, guessing will be too slow for the attacker to discover a
     strong passphrase.

     All of the hashing methods use a “salt” to perturb the hash function, so
     that the same passphrase may produce many possible hashes.  Newer methods
     accept longer salt strings.  The salt should be chosen at random for each
     user.  Salt defeats a number of attacks:

     1.   It is not possible to hash a passphrase once and then test it
          against each account's stored hash; the hash calculation must be
          repeated for each account.

     2.   It is not possible to tell whether two accounts use the same
          passphrase without successfully guessing one of the phrases.

     3.   Tables of precalculated hashes of commonly used passphrases must
          have an entry for each possible salt, which makes them impractically
          large.

     All of the hashing methods are also deliberately engineered to be slow;
     they use many iterations of an underlying cryptographic primitive to
     increase the cost of each guess.  The newer hashing methods allow the
     number of iterations to be adjusted, using the “CPU time cost” parameter
     to crypt_gensalt(3).  This makes it possible to keep the hash slow as
     hardware improves.

FORMAT OF HASHED PASSPHRASES
     All of the hashing methods supported by crypt(3) produce a hashed
     passphrase which consists of four components: prefix, options, salt, and
     hash.  The prefix controls which hashing method is to be used, and is the
     appropriate string to pass to crypt_gensalt(3) to select that method.
     The contents of options, salt, and hash are up to the method.  Depending
     on the method, the prefix and options components may be empty.

     The setting argument to crypt(3) must begin with the first three
     components of a valid hashed passphrase, but anything after that is
     ignored.  This makes authentication simple: hash the input passphrase
     using the stored passphrase as the setting, and then compare the result
     to the stored passphrase.

     Hashed passphrases are always entirely printable ASCII, and do not
     contain any whitespace or the characters ‘:’, ‘;’, ‘*’, ‘!’, or ‘\’.
     (These characters are used as delimiters and special markers in the
     passwd(5) and shadow(5) files.)

     The syntax of each component of a hashed passphrase is up to the hashing
     method.  ‘$’ characters usually delimit components, and the salt and hash
     are usually encoded as numerals in base 64.  The details of this base-64
     encoding vary among hashing methods.  The common “base64” encoding
     specified by RFC 4648 is usually not used.

AVAILABLE HASHING METHODS
     This is a list of all the hashing methods supported by crypt(3), in
     decreasing order of strength.  Many of the older methods are now
     considered too weak to use for new passphrases.  The hashed passphrase
     format is expressed with extended regular expressions (see regex(7)) and
     does not show the division into prefix, options, salt, and hash.

   yescrypt
     yescrypt is a scalable passphrase hashing scheme designed by Solar
     Designer, which is based on Colin Percival's scrypt.  Recommended for new
     hashes.

     Prefix
         "$y$"

     Hashed passphrase format
         \$y\$[./A-Za-z0-9]+\$[./A-Za-z0-9]{,86}\$[./A-Za-z0-9]{43}

     Maximum passphrase length
         unlimited

     Hash size
         256 bits

     Salt size
         up to 512 bits

     CPU time cost parameter
         1 to 11 (logarithmic)

   gost-yescrypt
     gost-yescrypt uses the output from the yescrypt hashing method in place
     of a hmac message.  Thus, the yescrypt crypto properties are superseeded
     by the GOST R 34.11-2012 (Streebog) hash function with a 256 bit digest.
     This hashing method is useful in applications that need modern passphrase
     hashing methods, but require to rely on the cryptographic properties of
     GOST algorithms.  The GOST R 34.11-2012 (Streebog) hash function has been
     published by the IETF as RFC 6986.  Recommended for new hashes.

     Prefix
         "$gy$"

     Hashed passphrase format
         \$gy\$[./A-Za-z0-9]+\$[./A-Za-z0-9]{,86}\$[./A-Za-z0-9]{43}

     Maximum passphrase length
         unlimited

     Hash size
         256 bits

     Salt size
         up to 512 bits

     CPU time cost parameter
         1 to 11 (logarithmic)

   scrypt
     scrypt is a password-based key derivation function created by Colin
     Percival, originally for the Tarsnap online backup service.  The
     algorithm was specifically designed to make it costly to perform large-
     scale custom hardware attacks by requiring large amounts of memory.  In
     2016, the scrypt algorithm was published by IETF as RFC 7914.

     Prefix
         "$7$"

     Hashed passphrase format
         \$7\$[./A-Za-z0-9]{11,97}\$[./A-Za-z0-9]{43}

     Maximum passphrase length
         unlimited

     Hash size
         256 bits

     Salt size
         up to 512 bits

     CPU time cost parameter
         6 to 11 (logarithmic)

   bcrypt
     A hash based on the Blowfish block cipher, modified to have an extra-
     expensive key schedule.  Originally developed by Niels Provos and David
     Mazieres for OpenBSD and also supported on recent versions of FreeBSD and
     NetBSD, on Solaris 10 and newer, and on several GNU/*/Linux
     distributions.

     Prefix
         "$2b$"

     Hashed passphrase format
         \$2[abxy]\$[0-9]{2}\$[./A-Za-z0-9]{53}

     Maximum passphrase length
         72 characters

     Hash size
         184 bits

     Salt size
         128 bits

     CPU time cost parameter
         4 to 31 (logarithmic)

     The alternative prefix "$2y$" is equivalent to "$2b$".  It exists for
     historical reasons only.  The alternative prefixes "$2a$" and "$2x$"
     provide bug-compatibility with crypt_blowfish 1.0.4 and earlier, which
     incorrectly processed characters with the 8th bit set.

   sha512crypt
     A hash based on SHA-2 with 512-bit output, originally developed by Ulrich
     Drepper for GNU libc.  Supported on Linux but not common elsewhere.
     Acceptable for new hashes.  The default CPU time cost parameter is 5000,
     which is too low for modern hardware.

     Prefix
         "$6$"

     Hashed passphrase format
         \$6\$(rounds=[1-9][0-9]+\$)?[./0-9A-Za-z]{1,16}\$[./0-9A-Za-z]{86}

     Maximum passphrase length
         unlimited

     Hash size
         512 bits

     Salt size
         6 to 96 bits

     CPU time cost parameter
         1000 to 999,999,999

   sha256crypt
     A hash based on SHA-2 with 256-bit output, originally developed by Ulrich
     Drepper for GNU libc.  Supported on Linux but not common elsewhere.
     Acceptable for new hashes.  The default CPU time cost parameter is 5000,
     which is too low for modern hardware.

     Prefix
         "$5$"

     Hashed passphrase format
         \$5\$(rounds=[1-9][0-9]+\$)?[./0-9A-Za-z]{1,16}\$[./0-9A-Za-z]{43}

     Maximum passphrase length
         unlimited

     Hash size
         256 bits

     Salt size
         6 to 96 bits

     CPU time cost parameter
         1000 to 999,999,999

   sha1crypt
     A hash based on HMAC-SHA1.  Originally developed by Simon Gerraty for
     NetBSD.  Not as weak as the DES-based hashes below, but SHA1 is so cheap
     on modern hardware that it should not be used for new hashes.

     Prefix
         "$sha1"

     Hashed passphrase format
         \$sha1\$[1-9][0-9]+\$[./0-9A-Za-z]{1,64}\$[./0-9A-Za-z]{8,64}[./0-9A-
         Za-z]{32}

     Maximum passphrase length
         unlimited

     Hash size
         160 bits

     Salt size
         6 to 384 bits

     CPU time cost parameter
         4 to 4,294,967,295

   SunMD5
     A hash based on the MD5 algorithm, with additional cleverness to make
     precomputation difficult, originally developed by Alec David Muffet for
     Solaris.  Not adopted elsewhere, to our knowledge.  Not as weak as the
     DES-based hashes below, but MD5 is so cheap on modern hardware that it
     should not be used for new hashes.

     Prefix
         "$md5"

     Hashed passphrase format
         \$md5(,rounds=[1-9][0-9]+)?\$[./0-9A-Za-z]{8}\${1,2}[./0-9A-Za-z]{22}


     Maximum passphrase length
         unlimited

     Hash size
         128 bits

     Salt size
         48 bits

     CPU time cost parameter
         4096 to 4,294,963,199

   md5crypt
     A hash based on the MD5 algorithm, originally developed by Poul-Henning
     Kamp for FreeBSD.  Supported on most free Unixes and newer versions of
     Solaris.  Not as weak as the DES-based hashes below, but MD5 is so cheap
     on modern hardware that it should not be used for new hashes.  CPU time
     cost is not adjustable.

     Prefix
         "$1$"

     Hashed passphrase format
         \$1\$[^$]{1,8}\$[./0-9A-Za-z]{22}

     Maximum passphrase length
         unlimited

     Hash size
         128 bits

     Salt size
         6 to 48 bits

     CPU time cost parameter
         1000

   bsdicrypt (BSDI extended DES)
     A weak extension of traditional DES, which eliminates the length limit,
     increases the salt size, and makes the time cost tunable.  It originates
     with BSDI and is also available on at least NetBSD, OpenBSD, and FreeBSD
     due to the use of David Burren's FreeSec library.  It is better than
     bigcrypt and traditional DES, but still should not be used for new
     hashes.

     Prefix
         "_"

     Hashed passphrase format
         _[./0-9A-Za-z]{19}

     Maximum passphrase length
         unlimited (ignores 8th bit)

     Hash size
         64 bits

     Effective key size
         56 bits

     Salt size
         24 bits

     CPU time cost parameter
         1 to 16,777,215 (must be odd)

   bigcrypt
     A weak extension of traditional DES, available on some System V-derived
     Unixes.  All it does is raise the length limit from 8 to 128 characters,
     and it does this in a crude way that allows attackers to guess chunks of
     a long passphrase in parallel.  It should not be used for new hashes.

     Prefix
         "" (empty string)

     Hashed passphrase format
         [./0-9A-Za-z]{13,178}

     Maximum passphrase length
         128 characters (ignores 8th bit)

     Hash size
         up to 1024 bits

     Effective key size
         up to 896 bits

     Salt size
         12 bits

     CPU time cost parameter
         25

   descrypt (Traditional DES)
     The original hashing method from Unix V7, based on the DES block cipher.
     Because DES is cheap on modern hardware, because there are only 4096
     possible salts and 2**56 possible hashes, and because it truncates
     passphrases to 8 characters, it is feasible to discover any passphrase
     hashed with this method.  It should only be used if you absolutely have
     to generate hashes that will work on an old operating system that
     supports nothing else.

     Prefix
         "" (empty string)

     Hashed passphrase format
         [./0-9A-Za-z]{13}

     Maximum passphrase length
         8 characters (ignores 8th bit)

     Hash size
         64 bits

     Effective key size
         56 bits

     Salt size
         12 bits

     CPU time cost parameter
         25

   NT
     The hashing method used for network authentication in some versions of
     the SMB/CIFS protocol.  Available, for cross-compatibility's sake, on
     FreeBSD.  Based on MD4.  Has no salt or tunable cost parameter.  Like
     traditional DES, it is so weak that any passphrase hashed with this
     method is guessable.  It should only be used if you absolutely have to
     generate hashes that will work on an old operating system that supports
     nothing else.

     Prefix
         "$3$"

     Hashed passphrase format
         \$3\$\$[0-9a-f]{32}

     Maximum passphrase length
         unlimited

     Hash size
         256 bits

     Salt size
         0 bits

     CPU time cost parameter
         1

SEE ALSO
     crypt(3), crypt_gensalt(3), getpwent(3), passwd(5), shadow(5), pam(8)

     Niels Provos and David Mazieres, "A Future-Adaptable Password Scheme",
     Proceedings of the 1999 USENIX Annual Technical Conference,
     https://www.usenix.org/events/usenix99/provos.html, June 1999.

     Robert Morris and Ken Thompson, "Password Security: A Case History",
     Communications of the ACM, 11, 22,
     http://wolfram.schneider.org/bsd/7thEdManVol2/password/password.pdf,
     1979.

Openwall Project               October 11, 2017               Openwall Project